Communication systems take many forms. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the message signal into a form suitable for transmission over the channel. This processing of the message signal is referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to process the received signal so as to produce an estimate of the original message signal. This processing of the received signal is referred to as demodulation.
One type of communication system is a spread-spectrum system. In a spread-spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth comparable to the bandwidth of the information itself. However, in a spread-spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal.
Three general types of spread-spectrum communication techniques exist, including direct sequence modulation, frequency and/or time hopping modulation, and chirp modulation. In direct sequence modulation, a carrier signal is modulated by a digital code sequence whose bit rate is much higher than the information signal bandwidth.
Information (i.e., the message signal consisting of voice and/or data) can be embedded in the direct sequence spread-spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code, because the combination of the spreading code and the information typically a binary code involves modulo-2 addition. Alternatively, the information or message signal may be used to modulate a carrier before spreading it.
These direct sequence spread-spectrum communication systems can readily be designed as multiple access communication systems. For example, a spread-spectrum system may be designed as a direct sequence code division multiple access (DS-CDMA) system. In a DS-CDMA system, communication between two communication units is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code. As a result, transmitted signals are in the same frequency band of the communication channel and are separated only by unique user spreading codes. These unique user spreading .codes preferably are orthogonal to one another such that the cross-correlation between the spreading codes is low (i.e., approximately zero).
Particular transmitted signals can be retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a user spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. Further, when the user spreading codes are orthogonal to one another, the received signal can be correlated with a particular user spreading code such that only the desired user signal related to the particular spreading code is enhanced while the other signals for all of the other users are de-emphasized.
It will be appreciated by those skilled in the art that several different spreading codes exist which can be used to separate data signals from one another in a DS-CDMA communication system. These spreading codes include but are not limited to pseudonoise (PN) codes and Walsh codes. A Walsh code corresponds to a single row or column of the Hadamard matrix.
Further it will be appreciated by those skilled in the art that spreading codes can be used to channel code data signals. The data signals are channel coded to improve performance of the communication system by enabling transmitted signals to better withstand the effects of various channel impairments, such as noise, fading, and jamming. Typically, channel coding reduces the probability of bit error, and/or reduces the required signal to noise ratio (usually expressed as bit energy per noise density i.e., E.sub.b /N.sub.o which is defined as the ratio of energy per information-bit to noise-spectral density), to recover the signal at the cost of expending more bandwidth than would otherwise be necessary to transmit the data signal. For example, Walsh codes can be used to channel code a data signal prior to modulation of the data signal for subsequent transmission. Similarly PN spreading codes can be used to channel code a data signal.
However, channel coding alone may not provide the required signal to noise ratio for some communication system designs which require the system to be able to handle a particular number of simultaneous communications (all having a minimum signal to noise ratio). This design constraint may be satisfied, in some instances, by designing the communication system to coherently detect transmitted signals rather than using non-coherent reception techniques. In coherent detection systems, the channel response is determined so that the affects of phase and magnitude distortions caused by the communication channel can be compensated for with matched filters. In contrast, non-coherent detection systems typically do not compensate for the phase distortion in a received signal which was caused by the communication channel. It will be appreciated by those skilled in the art that a coherent receiver requires less signal to noise ratio (in E.sub.b /N.sub.o) than that required by a non-coherent receiver having the same bit error rate (i.e., a particular design constraint denoting an acceptable interference level). Roughly speaking, there is a three deciBel (dB) difference between them for the Raleigh fading channel. The advantage of the coherent receiver is more significant when diversity reception is used, because there is no combining loss for an optimal coherent receiver while there is always a combining loss for noncoherent receiver.
One such method for facilitating coherent detection of transmitted signals is to use a pilot signal. For example, in a cellular communication system the forward channel, or down-link, (i.e., from base station to mobile unit) may be coherently detected if the base station transmits a pilot signal. Subsequently, all the mobile units use the pilot channel signal to estimate the channel phase and magnitude parameters. However, for the reverse channel, or up-link, (i.e., from mobile to base station), using such a common pilot signal is not feasible. As a result, those of ordinary skill in the art often assume that only non-coherent detection techniques are suitable for up-link communication. As a result, many recent publications have focused on optimizing noncoherent reception in DS-CDMA systems. Ideally a communication system should be designed to coherently received DS-CDMA signals.
However, channel coding and coherent detection still may not provide the required signal to noise ratio for some communication system designs which require the system to be able to handle a particular number of simultaneous communications (all having a minimum signal to noise ratio). It will be appreciated by those skilled in the art that in spread-spectrum CDMA communication systems the noise interference level in the communication channel is directly related to number of simultaneous but code divided users within the communication channel. Thus, in order to reduce the noise interference, the number of simultaneous code divided users in the communication channel is typically limited.
In addition, the noise interference is also affected by the received signal power level. In some spread-spectrum communication systems (e.g., cellular systems) a central communication site typically attempts detect or receive more than one signal from a particular band of the electromagnetic frequency spectrum. The central communication site adjusts the receiver components to optimally receive signals at a particular received signal power threshold value. Those received signals having a received signal power level at or near the particular power threshold level are optimally received. While those received signals not having a received signal power level at or near the particular power threshold level are not optimally received. A non-optimally received signal tends to have a higher bit error rate and tends to cause unnecessary interference to other receivers. Either of these undesirable consequences of a non-optimally received signal can result in the communication system further limiting the number of simultaneous users in the communication channel associated with the central communication site.
Thus, it is desirable to maintain the received signal power level at or near the particular power threshold level. This can be accomplished by adjusting the signal power level of transmitters attempting to transmit to the central communication site. Therefore, by using power control schemes to maintain the received signal power levels at a particular power threshold level the number of simultaneous users in a communication channel can be maximized for a particular maximum error rate limit. This is especially important when vehicle speed is low. To achieve effective power control, an unbiased and accurate power estimate is needed. For non-coherent detection techniques in a DS-CDMA communication system, the use of orthogonal symbols, either bit interleaved or orthogonal symbol interleaved, is power estimation methods have been proposed, However, a need exists for a more accurate received signal power level estimation scheme for use in a coherent DS-CDMA spread-spectrum communication system. Through the use of a more accurate received signal power level estimation scheme, the number of simultaneous users in a communication channel can be increased over the number of simultaneous users in a communication channel using less accurate received signal power level estimation scheme while maintaining the same maximum error rate limit.